Wireless communication systems are widely deployed to provide various communication services such as voice, packet data, broadcast, and so on. These systems may be capable of providing communication for multiple users simultaneously by sharing the available system resources. Some examples of such systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, and frequency division multiple access (FDMA) systems.
A wireless communication system may provide broadcast service, which typically entails the transmission of broadcast data to users in a designated broadcast area instead of specific users. Since a broadcast transmission is intended to be received by multiple users within the broadcast area, the broadcast data rate is normally determined by the user with the worst channel conditions. Typically, the worst-case user is located far away from a transmitting base station and has a low signal-to-noise ratio (SNR).
The users in the broadcast area typically experience different channel conditions, achieve different SNRs, and are capable of receiving data at different data rates. Hierarchical transmission may then be used to improve broadcast service. With hierarchical transmission, the broadcast data is divided into a “base stream” and an “enhancement stream”. The base stream is transmitted in a manner such that all users in the broadcast area can recover this stream. The enhancement stream is transmitted in a manner such that users experiencing better channel conditions can recover this stream. Hierarchical transmission is also referred to as hierarchical coding, where the term “coding” in this context refers to channel coding rather than data coding at the transmitter.
One conventional method of implementing hierarchical coding is through the use of non-uniform modulation. In this method, data for the base stream is modulated with a first modulation scheme and data for the enhancement stream is modulated with a second modulation scheme that is superimposed on the first modulation scheme. The first modulation scheme is typically a low-order modulation scheme such as QPSK, and the second modulation scheme may also be QPSK. In this case, the resultant modulated data for both streams may resemble 16-QAM modulated data. All users in the broadcast area are able to recover the base stream using QPSK demodulation. Users with better channel conditions are also able to recover the enhancement stream by removing the modulation due to the base stream. Hierarchical coding using non-uniform modulation is implemented by some conventional systems such as a Direct Video Broadcasting standard T (DVB-T) system.
Hierarchical coding is conventionally used for a single-input single-output (SISO) system. A SISO system employs a single antenna at a transmitter and a single antenna at a receiver. For the SISO system, hierarchical coding may be implemented, for example, using non-uniform modulation as described above.
A wireless communication system may employ multiple antennas at either the transmitter or the receiver, or at both the transmitter and receiver. The multiple antennas may be used to provide diversity against deleterious path effects and/or to improve transmission capacity, both of which are desirable. There is a need in the art for techniques to perform hierarchical coding with multiple antennas in a wireless communication system.